The transmission of radio signals in modern wireless communications can be realized based on a number of different communications systems, often specified by a standard. There are increasing requirements for devices which are able to operate to support more than one of these wireless communications systems. Mobile radio receiver devices include analog radio frequency (RF)/intermediate frequency (IF) stages, which are arranged to receive and transmit wireless signals via one or more antennas. The output of the RF/IF stages is typically converted to baseband, where an Analog-to-Digital Converter (ADC) converts incoming analog signals to digital samples, which are then processed for signal detection and decoding of the information data. The ADC may alternatively operate directly at IF, in which case the conversion to baseband is performed in the digital domain. A number of different types of front end processing of the digital samples are known to implement signal detection, including rake receiver processing and channel equalisation processing.
In Code-Division Multiple Access (CDMA) wireless systems, different physical channels are multiplexed in the code domain using separate spreading sequences. In the case of orthogonal spreading codewords, the original data symbols can then be effectively separated at the receiver by despreading.
In a Wideband CDMA (WCDMA) cellular system, downlink code multiplexing is performed using Orthogonal Variable Spreading Factor (OVSF) codes. However, the OVSF codewords are orthogonal to each other only under the condition of perfect time alignment. In the presence of multipath propagation, the code orthogonality is lost, and the operation of despreading is affected by Multiple Access Interference (MAI).
CDMA mobile radio receivers conventionally employ a rake processor which relies on the correlation properties of the spreading sequences. A rake processor is described for example in J. G. Proakis, “Digital Communications”, New York: McGraw-Hill, 1995. This type of receiver is subject to performance degradation in the presence of code correlation, if the MAI between code-multiplexed transmission is comparable to the other sources of noise and interference. Under these conditions, a performance advantage may be achieved by attempting to restore the orthogonality between the codes before despreading. The sub-optimality of conventional 3GPP receivers based on rake processing causes a significant performance penalty, especially for downlink data rates increasing from the 384 kbps for WCDMA Release 99 to High Speed Downlink Packet Access (HDSPA) rates of several Mbps. When the code orthogonality is destroyed by multipath, an effective approach is to use channel equalisation instead of rake processing.
Channel equalisation techniques have been widely employed over the last decades for combating intersymbol interference on frequency selective transmission channels. Channel equalisation techniques are described in J. G. Proakis, “Digital Communications”, New York: McGraw-Hill, 1995, and S. Benedetto, E. Biglieri, and V. Castellani, “Digital Transmission Theory”, Englewood Cliffs, N.J.: Prentice-Hall, 1987. Channel equalisers have recently found application in receivers for Time Division Multiple Access (TDMA) and code division multiple access (CDMA) mobile wireless systems. An example of application of channel equalisation to a CDMA cellular system is described in A. Klein “Data Detection Algorithms Specially Designed for the Downlink of CDMA Mobile Radio Systems”, IEEE Vehicular Technology Conference, vol. 1, Phoenix Ariz., May 1997, pp. 203-207. In particular in asynchronous CDMA cellular systems, as in the case of the forward link of the 3GPP WCDMA standard, chip level equalisation allows to significantly improve the performance over conventional rake receivers, at the cost of an increased implementation complexity. This advantage is especially important for high rate data transmission, as in 3GPP high speed downlink packet access (HDDPA).
However, channel equalization may not be able to provide superior performance in all possible scenarios. In particular, the use of a channel equaliser does not provide an advantage under single-ray propagation conditions, i.e., in the absence of multipath propagation.
The above limitations generally depend on the particular equalization algorithm under consideration. In the case of a linear MMSE equaliser, in the presence of a non-frequency selective or flat channel response, the equaliser processing still relies on the estimation of the channel impulse response, with a channel estimation error proportional to the number of the channel impulse response samples. In this situation, the use of a rake receiver not only does not correspond to a performance loss caused by MAI, but in fact reduces to a minimum the channel estimation error, relying on the estimate of a single channel tap.
Similarly, in the case of a Least-Squares (LS) equaliser, the receiver performance may be penalized by using the estimation of the channel statistics performed with a dimensionality higher than required in the specific conditions of non dispersive channel, i.e., of channel propagation profile with a single tap.
It is an aim of the present invention to optimise the processing facilities of a receiver in a wireless communication environment, in particular taking into account required signal processing performance set against the computing resources and/or power consumption required to obtain that processing performance.
It is another aim of this invention to identify n-ray propagation conditions, which is capable of resolving the above issue.